1. Field of the Invention
The invention relates to a method of assaying an element in a material comprising silicon, the material being intended in particular for manufacturing components for optics, electronics, or optoelectronics. For example, the invention can be used for assaying metal elements that might contaminate silicon substrates.
2. Background of the Invention
In silicon technology, the essential component in microelectronics is the metal-oxide-semiconductor (MOS) capacitor. The structure of a MOS capacitor comprises two electrodes (e.g., one made of metal or of polycrystalline silicon, and the other of monocrystalline silicon), separated by a fine insulating layer of silicon oxide (SiO2) referred to as the “dielectric” or the “grid oxide”. The grid oxide is made by thermal oxidation, with growth taking place on the surface and in depth. Thermal oxides are vitreous (or amorphous) materials that do not present long-distance crystallographic order, but merely chemical uniformity and short-distance order. Only 43% of the space in the crystal lattice is occupied, thus encouraging trapping and diffusion mechanisms. The presence of impurities gives rise to foreign species which occupy sites that are interstitial or substitutional. These impurities are mainly alkali ions (calcium, potassium, sodium) and metal ions (iron, aluminum, zinc, copper, nickel, . . . ).
If a substrate presenting such impurities in the active silicon layer is raised to high temperature, then the impurities can precipitate and thus lead to crystal defects forming.
Impurity diffusion gives rise to segregation of species and to localized high concentrations. These act as traps for charge carriers, providing energy levels that facilitate electron-hole recombinations. Such impurities degrade electrical performance, or can lead to dielectric breakdown in future circuits. Nowadays, the thickness of grid oxides can be as little as a few tens of angstroms (Å). For example, a layer which is 40 Å thick corresponds to about 20 layers of atoms. This small thickness exacerbates the impact of any defects. As a result, the quality of such a layer is intimately associated with the oxidation method and also with the quality of the substrate on which it is grown. In certain microelectronic applications, an oxide layer is formed beneath a semiconductor layer. Such an oxide layer is referred to as a “buried” oxide.
Buried oxides are used, for example, when making silicon on insulator (SOI) substrates. Substrates of that type can be used to make an electronic circuit on silicon insulated from a mechanical support by insulation, in this case the buried oxide. In a buried oxide, the presence of contaminants degrades electrical insulation between the circuit and the mechanical support. It is therefore important to assay such impurities.
Furthermore, numerous instruments made of a material comprising a high concentration of silicon, such as glass, quartz, etc., are used in methods of manufacturing substrates and components for microelectronics, optics, or optoelectronics. The performance of such substrates and such components depends in particular on the impurities they contain. Unfortunately, the instruments with which they are handled (such as quartz boats for substrates, for example) can contaminate them. Thus, in this case also, it can be desirable to assay the contaminants in such instruments.
Numerous techniques are already known for assaying elements, such as impurities, in a material.
Some of those techniques do not require the sample to be prepared. By way of example, those techniques comprise secondary ion mass spectrometry (SIMS) and total X-ray fluorescence (TXRF).
Other techniques require a sample to be prepared, for example merely by being put into solution. That applies in particular to atomic absorption spectrometry (AAS) and sometimes also to TXRF.
When samples are being prepared for an assay, it is necessary to avoid adding contaminants (operational contamination) or removing contaminants (precipitation, adsorption, etc.).
Known techniques of preparing samples for such assays consist in chemically etching the surface of a substrate.
One of those techniques is known to the person skilled in the art as vapor phase decomposition (VPD). Another of those techniques is known to the person skilled in the art as liquid phase decomposition (LPD).
The VPD techniques consists in etching the silicon oxide with hydrofluoric acid vapor. That reaction is generally performed at ambient temperature and it can be facilitated by cooling the material that is to be etched in order to encourage hydrofluoric acid vapor to condense on the material.
Etching occurs in accordance with the following reaction:SiO2(s)+6HF(aq)→H2SiF6(aq)+2H2O  (1)
That technique can also be used for decomposing native oxide, typically several tens of angstroms thick, on silicon or even thicker layers of oxide (e.g. several hundred angstroms thick) that have been deliberately formed, for example during thermal oxidation.
The LPD technique can be used both to etch silicon and to etch silicon oxide.
This technique can be used to etch oxides that are thick, typically several thousand angstroms thick to a few microns thick, or indeed a few tens of microns thick, e.g. as obtained by oxide deposition, with this being done by using a solution of hydrofluoric acid (HF).
Still using this technique, in order to etch silicon that has not been oxidized, for example, a mixture is used of nitric acid (HNO3) and of hydrofluoric acid (HF). The reaction on silicon is then as follows:Si+HNO3+6HF→H2SiF6+NO2+H2O+H2
During such etching, several reactions take place simultaneously: the silicon is oxidized by the nitric acid and the silicon oxide is etched by the hydrofluoric acid.
In those two etching techniques, in vapor phase and in liquid phase, the various elements initially present in the silicon oxide or in the silicon are also passed into solution.
Assaying techniques other than AAS and/or TXRF cannot make do with those decomposition techniques only. This applies, for example, with inductively coupled plasma mass spectrometry (ICPMS), which has the particular advantage of being considerably faster than the above-cited techniques. The solution obtained after such preparatory steps, whether in liquid phase or in vapor phase, and when the thickness to be etched is large, has a silicon concentration that is typically equal to several grams per liter, which can represent a concentration that is too high for present-day ICPMS equipment (saturation of the analyzer, in particular concerning ionization of elements). The viscosity of the resulting solution is also incompatible with present-day ICPMS equipment. Under such circumstances, more advanced techniques are required for preparing the sample.
Thus, there is a need for improved assaying techniques to accurately determine impurities in such materials.